The Microwave Radiometer Technology Acceleration (MiRaTA) satellite, a 3U CubeSat, is shown with solar panels fully deployed, flanking the body of the spacecraft, which has a circular aperture at the top for the microwave radiometer antenna, used for atmospheric science measurements. There are also two small, thin tape-measure antennas on the top, used for UHF radio communication with the ground station. Courtesy of MIT Lincoln Laboratory.

MiRaTA is designed to demonstrate that a small satellite can carry instrument technology that's capable of reducing the cost and size of future weather satellites and has the potential to routinely collect reliable weather data. Microwave radiometers are one of the workhorse instruments aboard today's weather satellites. These sensitive instruments measure radio frequency signals related to the thermal radiation emitted by atmospheric gases, such as molecular oxygen and water vapor, and also detect particles such as cloud ice. These data are key inputs for models that track storms and other weather events.

Calibrating these radiometers is important for keeping them from drifting so their data can be used for accurate weather and climate models. Therefore, a calibration target is usually included in the satellite to help the radiometer maintain its accuracy. Miniaturizing microwave radiometer instruments to fit on a CubeSat leads to the challenge of finding a calibration instrument that is not only accurate but also compact, according to Kerri Cahoy, principal investigator for MiRaTA and an associate professor in the Department of Aeronautics and Astronautics at the Massachusetts Institute of Technology.

"You don't have room for the bulky calibration targets that you would normally use on larger satellites," Cahoy said. "Microwave radiometer calibration targets on larger satellites can be the size of a toaster, but for CubeSats, it would have to be the size of a deck of cards."

Cahoy and her colleague William Blackwell, the microwave radiometer instrument lead at MIT Lincoln Laboratory, have come up with a solution based on a technique she studied in graduate school called radio occultation (RO), whereby radio signals received from GPS satellites in a higher orbit are used to measure the temperature of the same volume of atmosphere that the radiometer is viewing. The GPS-RO temperature measurement can then be used for calibrating the radiometer.

"In physics class, you learn that a pencil submerged in water looks like it's broken in half because light bends differently in the water than in the air," Cahoy said. "Radio waves are like light in that they refract when they go through changing densities of air, and we can use the magnitude of the refraction to calculate the temperature of the surrounding atmosphere with near-perfect accuracy and use this to calibrate a radiometer."

"Our goal is to have our radiometers perform just as well as those on current weather satellites and be able to provide the kind of data that helps agencies and people in the path of a natural disaster prepare early and wisely," Cahoy said.